Organic thin film transistors (OTFTs) are devices that use a gate electrode to control current flow between a source electrode and a drain electrode under an imposed bias. A serious problem affecting OTFTs relates to their high sensitivity to oxygen and water in the atmosphere. Exposure of OTFTs to the atmosphere degrades their performance. Pentacene OTFTs, for example, become doped upon exposure to oxygen, and reaction with water changes the lattice structure. Light enhances these effects.
Attempts have been made at solving the degradation problems using chemical barrier layers. The barrier layers should resist penetration by air and water. They should be mechanically robust, easily handled, stable over time, and should maintain its permeability properties over time. For example, the components have been sealed inside glass, or protected from the atmosphere using a multilayer barrier of inorganic oxide separated by polymer. Although the results obtained with the multilayer barrier of inorganic oxide separated by polymer look promising, the additional thickness from a few hundred nanometers to 5-7 micrometers seriously affects the flexibility of the device. Another attempt at solving the degradation problem involves encapsulation with a TiOx layer that acts both as a shielding and oxygen scavenging layer. The carrier mobility (μ) of this TiOx-encapsulated device was stable in the atmosphere (i.e. in the air) for up to about 500 hours but subsequently degraded about one order of magnitude after 2000 hours.
In addition to the degradation problems, a large contact resistance RC typically exists between the organic active layer and the electrodes. Methods to reduce RC have been proposed that are based on the introduction of self assembled monolayers at the metal/organic interface or on chemical treatment of the metal electrode prior to the organic deposition. Such methods are limited because they are demanding to implement for large scale reliable fabrication processes.
Providing organic thin film devices with improved electronic properties and an extended operating lifetime in an atmospheric environment is desirable.